The last family forms at or near the surface — no deep magma or fluids required.

• Laterite (weathering) — in hot, wet climates, deep weathering rots rock and leaves metal concentrated in the soil, giving nickel laterites and bauxite (aluminium). Weathering can also enrich the top of a porphyry — dissolving copper and re-depositing it richer just below the surface, called supergene enrichment.
• Placer (gravity) — rivers and waves wash away light sand and concentrate heavy, durable grains: placer gold, titanium-zircon sands, even diamonds. South Africa's giant Witwatersrand goldfield — the source of much of the gold ever mined — is an ancient buried placer (a paleoplacer).
• Sedimentary — some of the biggest ore bodies are chemical sediments. Banded iron formations (BIF) laid down in ancient oceans hold most of the world's iron. Evaporites give potash and salt (Unit 2). And sediment-hosted copper forms giant deposits like the Central African Copperbelt.

The big picture — one crust, many deposits

Almost every deposit is one of a few engines acting in a setting. Read a deposit's setting and host rock, and you can usually name its type — and predict its metals.

The section below shows the intrusion-and-surface cast of characters together. A few major models sit in their own settings outside it: IOCG iron-oxide systems, invisible Carlin-type gold in sedimentary rock, uranium at unconformities and roll-fronts, diamonds in kimberlite pipes, and granite-related tin and tungsten — but the same rule names them all: setting plus host rock.